Serum chemistry alterations in Alaskan sled dogs during five successive days of prolonged endurance exercise

Erica C. McKenzie Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078

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Eduard Jose-Cunilleras Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210

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Kenneth W. Hinchcliff Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210

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Todd C. Holbrook Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078

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Christopher Royer Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078

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Mark E. Payton Department of Statistics, College of Arts and Sciences, Oklahoma State University, Stillwater, OK 74078

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Kathy Williamson Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078

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Stuart Nelson Iditarod Trail Committee, Mile 2.2 Knik Goose Bay Rd, Wasilla, AK 99654

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Michael D. Willard Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843

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Michael S. Davis Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078

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Abstract

Objective—To determine the impact of successive days of endurance exercise on select serum chemistry values in conditioned Alaskan sled dogs.

Design—Prospective cohort study.

Animals—10 conditioned Alaskan sled dogs.

Procedures—All dogs ran 160 km/d for 5 consecutive days. Serum was obtained prior to exercise and immediately after each exercise run; all samples were obtained before dogs were fed. Serum electrolyte, mineral, protein, total bilirubin, urea nitrogen, creatinine, and cardiac troponin-I concentrations and serum alkaline phosphatase, alanine aminotransfer-ase, creatine kinase, and aspartate aminotransferase activities were measured. Data were analyzed by means of analysis of covariance for a randomized complete block design with dog as a blocking variable, time as a covariate, and distance run as the treatment of interest. Least square mean values were compared with values obtained prior to exercise, and linear and quadratic contrasts were examined.

Results—Serum globulin concentration was low prior to exercise (mean ± SD, 2.2 ± 0.3g/dL) and progressively decreased as exercise continued. Exercise was associated with increases in serum chloride, urea nitrogen, and cardiac troponin-I concentrations and serum alanine aminotransferase, creatine kinase, and aspartate aminotransferase activities and with pro-gressive decreases in serum potassium, total protein, and albumin concentrations.

Conclusions and Clinical Relevance—Results suggested that multiple successive days of endurance exercise resulted in mild aberrations in serum chemistry variables in conditioned sled dogs. Changes likely reflected the metabolic stresses of prolonged endurance exercise as well as dietary composition. Hypoglobulinemia in resting, conditioned sled dogs may reflect the immunosuppressive or catabolic effects of intense endurance training.

Abstract

Objective—To determine the impact of successive days of endurance exercise on select serum chemistry values in conditioned Alaskan sled dogs.

Design—Prospective cohort study.

Animals—10 conditioned Alaskan sled dogs.

Procedures—All dogs ran 160 km/d for 5 consecutive days. Serum was obtained prior to exercise and immediately after each exercise run; all samples were obtained before dogs were fed. Serum electrolyte, mineral, protein, total bilirubin, urea nitrogen, creatinine, and cardiac troponin-I concentrations and serum alkaline phosphatase, alanine aminotransfer-ase, creatine kinase, and aspartate aminotransferase activities were measured. Data were analyzed by means of analysis of covariance for a randomized complete block design with dog as a blocking variable, time as a covariate, and distance run as the treatment of interest. Least square mean values were compared with values obtained prior to exercise, and linear and quadratic contrasts were examined.

Results—Serum globulin concentration was low prior to exercise (mean ± SD, 2.2 ± 0.3g/dL) and progressively decreased as exercise continued. Exercise was associated with increases in serum chloride, urea nitrogen, and cardiac troponin-I concentrations and serum alanine aminotransferase, creatine kinase, and aspartate aminotransferase activities and with pro-gressive decreases in serum potassium, total protein, and albumin concentrations.

Conclusions and Clinical Relevance—Results suggested that multiple successive days of endurance exercise resulted in mild aberrations in serum chemistry variables in conditioned sled dogs. Changes likely reflected the metabolic stresses of prolonged endurance exercise as well as dietary composition. Hypoglobulinemia in resting, conditioned sled dogs may reflect the immunosuppressive or catabolic effects of intense endurance training.

In dogs and humans, prolonged endurance exercise results in catabolism of endogenous energy stores, skeletal muscle damage, alterations in hydration, and disturbances in electrolyte concentrations and acidbase status.1–4 Additionally, transient increases in serum cTnI concentration have been documented in elite human athletes undergoing intense endurance exercise, potentially representing cardiac stress or injury.5,6 Alaskan sled dogs might be expected to be at particular risk of developing biochemical aberrations during endurance exercise because energy expenditure during endurance exercise in sled dogs has been calculated to greatly exceed that of elite human athletes undergoing consecutive days of prolonged exercise.2 Furthermore, when running as a team, sled dogs are able to cover distances on consecutive days that exceed distances individuals of other species are capable of covering when running at an equivalent speed.

In contrast to other athletic species (eg, humans and horses), dogs are unique because they do not sweat to any appreciable extent. Approximately 40% of the heat load in exercising dogs is dissipated via convection and radiation, with the remainder dissipated via the respiratory tract.7 Individuals of species that depend heavily on evaporative heat loss via sweating (eg, horses) can incur substantial fluid and electrolyte losses during exercise of relatively limited duration.8 Thus, prolonged endurance exercise in dogs might produce alterations in serum chemistry variables that differ in type or severity from those documented during prolonged exercise in other species.

Both training and racing have been reported to induce specific alterations in serum protein, electrolyte, mineral, urea nitrogen, creatinine, and hormone concentrations and serum enzyme activities in sled dogs.9–13 However, previous studies1,10,14 of racing sled dogs have been complicated by a limited ability to obtain repeated samples from the same individuals; limited access to dogs during the immediate postexercise period, leading to delays of several hours prior to sample collection or collection of samples after dogs have been fed; and variability in the distances run per day. Furthermore, the impact of prolonged exercise on cTnI concentrations in dogs has not been reported, though cTnI concentrations have been demonstrated to increase with cardiac disease in dogs.15

The purpose of the study reported here, therefore, was to determine the impact of successive days of endurance exercise on select serum chemistry values, including serum cTnI concentration, in conditioned Alaskan sled dogs. The simulated endurance race format that was used allowed repeated collection of samples from 10 conditioned sled dogs that ran 160 km under relatively controlled conditions each day for 5 successive days.

Materials and Methods

Dogs—Ten conditioned Alaskan sled dogs from a single dog yard were used in the study. Dogs ranged from 2 to 7.4 years old (mean, 4.4 years). There were 6 males and 4 females. Mean ± SD body weight was 22.5 ± 2.8 kg (49.6 ± 6.2 lb).

All dogs were in training for multiday endurance racing at the time of the study. Dogs had been exercised 3 to 4 days each week for approximately 5 months prior to the study; maximum distance run during this time was 80 km (50 miles). Dogs were rested from training for 3 days prior to the start of the present study. Dogs were typically housed in individual enclosed kennels lined with straw. But during the present study, dogs were individually housed in a snow-covered yard with straw bedding.

Experimental design—All procedures were approved by the Oklahoma State University Institutional Animal Care and Use Committee and performed in accordance with principles outlined in the NIH Guide for the Care and Use of Laboratory Animals. The study was performed during January in Northern Alaska. Dogs were weighed prior to and at the end of the study.

The 10 dogs included in the present study represented a subset of the 36 dogs included in a related study.16 Each day for 5 successive days, the 36 dogs were allotted to teams of 10 to 18 dogs each and exercised for 160 km. During these exercise runs, dog teams pulled similarly weighted, lightly laden sleds and a musher. Each team traveled 80 km in approximately 5 hours to a remote location where dogs were rested for 7 to 8 hours before running the 80-km return journey. Dogs were allowed to rest for 6 to 8 hours before the commencement of the next day's exercise run. The 10 dogs included in the present study represented those remaining after the fifth exercise run (ie, after a cumulative distance of 800 km) following removal of 6 randomly selected dogs after the end of each previous day's exercise run (ie, after dogs had run cumulative distances of 160, 320, 480, and 640 km) and the premature removal of 2 dogs because of musculoskeletal problems.16

Throughout the study, all dogs consumed a similar diet estimated to provide approximately 50% of daily digestible energy intake as fat and less than 15% of daily digestible energy intake as carbohydrate.16 The diet was designed by the musher and consisted of a mixture of beef, fish, liver, tripe, commercial dried dog food,a-c and a commercial fat supplement.d Dogs were fed individually at the start and finish of each 6- to 8-hour rest period throughout the study (ie, dogs were fed 4 meals each 24 hours). Each meal provided 540 to 2,850 kcal depending on the time of day when the meal was fed, with the largest meal given at the completion of each 160-km exercise run.16 Water intake was provided solely by water mixed into the food, providing approximately 2.5 L of water/d, although dogs also had access to snow and ice in their surroundings.

Sample collection and analysis—Blood samples (8 to 10 mL) were collected from each dog by means of jugular venipuncture into plain evacuated glass tubes prior to the start of the study and within 20 to 60 minutes of the completion of each day's 160-km exercise run. All samples were collected prior to food intake. Blood samples were centrifuged within 30 minutes of collection, and the serum fraction was removed and frozen at −20°C until analyzed.

Serum samples were analyzed with an automated chemistry analyzere within 48 hours of blood sample collection. Analyses included measurement of serum sodium, potassium, chloride, calcium, phosphorus, urea nitrogen, creatinine, total bilirubin, total protein, and albumin concentrations and serum CK, AST, ALP, and ALT activities. Serum globulin concentration was calculated by subtracting measured serum albumin concentration from measured serum total protein concentration for each sample. For most variables, reference ranges for resting sled dogs had been established by the laboratory that processed the blood samples.f Reference ranges for serum albumin and globulin concentrations in resting sled dogs were not available; therefore, reference ranges established for nonathletic dogs were used.17

Serum cTnI concentrations were determined within 3 months after blood sample collection by means of a chemiluminescence immunoassay.g Sensitivity of the assay was 0.1 ng/mL. Interassay coefficient of variation was 5.2%, and intra-assay coefficient of variation was 1.1% for samples with a concentration of 1 ng/mL and 3.8% for samples with a concentration of 18.8 ng/mL. A reference value established for healthy resting dogs was used.15

Statistical analysis—Data were analyzed by means of analysis of covariance for a randomized complete block design with dog as a blocking variable, time as a covariate, and distance run (ie, 0, 160, 320, 480, 640, and 800 km) as the treatment of interest. Linear and quadratic contrasts were fit, and corresponding regression analyses were performed. The quadratic model was selected in favor of a linear model when the quadratic model was associated with a P value ≤ 0.05. Least square mean values were calculated (adjusted to the mean time for each dog) and compared with values obtained prior to exercise (ie, 0 km). Data for serum CK activity exhibited heteroskedasticity, so values were log transformed prior to analysis. All analyses were performed with commercially available software.h Values of P < 0.05 were considered significant.

Results

Body weight—Mean ± SD weight loss during the 5-day study period was 0.53 ± 0.46 kg (1.16 ± 1.02 lb). Eight dogs lost weight during the study, while 2 dogs each gained a small amount of weight.

Serum sodium, potassium, and chloride concentrations—Exercise was associated with significant but relatively minor changes in serum sodium (P = 0.022) and potassium (P = 0.003) concentrations (Table 1). Serum sodium concentration increased with exercise, and mild hypernatremia with serum sodium concentrations up to 155 mEq/L was observed in several dogs after the first (160 km) and fourth (640 km) days of exercise. Serum potassium concentration gradually decreased throughout the study and was significantly lower after 5 days of exercise (800 km) than it had been prior to the start of the study (0 km). However, in all dogs, serum potassium concentration was within reference limits throughout the study.

Table 1—

Serum chemistry values in 10 Alaskan sled dogs that exercised every day for 5 consecutive days by running 160 km/d.

VariableDistance traveled (km)Reference rangeR2
0160320480640800
Sodium (mEq/L)*148 ± 2153 ± 2150 ± 3151 ± 3149 ± 2149 ± 2144–1530.3937
Potassium (mEq/L)4.8 ± 0.34.6 ± 0.24.9 ± 0.34.7 ± 0.34.4 ± 0.44.3 ± 0.33.7–5.40.6124
Calcium (mg/dL)*10.3 ± 1.69.9 ± 0.49.4 ± 0.29.5 ± 0.39.2 ± 0.29.3 ± 0.39.0–11.10.5025
Phosphorus (mg/dL)5.1 ± 0.56.2 ± 0.65.0 ± 0.65.4 ± 0.65.0 ± 0. 74.7 ± 0.481.9–6.40.2461
SUN (mg/dL)*16 ± 246 ± 642 ± 1040 ± 942 ± 639 ± 64–260.6632
Creatinine (mg/dL)*0.6 ± 0.11.0 ± 0.20.9 ± 0.20.8 ± 0.20.8 ± 0.10.8 ± 0.10.4–1.50.6177
Albumin (g/dL)*3.9 ± 0.23.5 ± 0.23.1 ± 0.23.2 ± 0.33.0 ± 0.23.2 ± 0.32.3–3.10.9454
Globulin (g/dL)2.2 ± 0.31.9 ± 0.31.9 ± 0.31.7 ± 0.31.7 ± 0.41.6 ± 0.42.7–4.40.8821
ALP (U/L)*25 ± 2173 ± 23160 ± 2254 ± 2062 ± 2653 ± 2710–1290.7690
ALT (U/L)44 ± 1678 ± 2198 ± 27115 ± 40132 ± 46135 ± 2919–700.9105
AST (U/L)*43 ± 6138 ± 94220 ± 131233 ± 167172 ± 79163 ± 8115–430.9380

Significant (P < 0.05) quadratic contrast.

Significantly (P < 0.05) different from value obtained prior to exercise (0 km).

Significant (P < 0.05) linear contrast.

Serum chloride concentration increased significantly (P < 0.001) with exercise, increasing an average of 9 mEq/L after the first day's exercise (160 km) and remaining high all subsequent days (Figure 1). After the second, third, fourth, and fifth days of exercise (320, 480, 640, and 800 km), between 1 and 3 dogs had hyperchloremia, with serum chloride concentrations in these dogs ranging from 125 to 127 mEq/L (reference range, 109 to 123 mEq/L).

Figure 1—
Figure 1—

Mean ± SD serum chloride concentration (reference range, 109 to 123mEq/L) in 10 Alaskan sled dogs that exercised every day for 5 consecutive days by running 160 km/d. Values obtained after 160, 320, 480, 640, and 800 km were significantly (P < 0.05) different from value obtained at time 0.

Citation: Journal of the American Veterinary Medical Association 230, 10; 10.2460/javma.230.10.1486

Serum calcium and phosphorus concentrations—� Mean serum calcium concentration was within reference limits at all times during the study but was significantly (P = 0.003) decreased after the second day's exercise (320 km), compared with mean concentration before exercise (Table 1). Minimal changes were observed in serum phosphorus concentration. However, 5 of the 10 dogs had hyperphosphatemia after the first day's exercise (160 km), with serum phosphorus concentrations in these dogs ranging from 6.6 to 7.0 mg/dL.

Serum protein concentration—After each day's exercise, mean serum total protein concentration was significantly (P < 0.001) decreased, compared with mean concentration prior to exercise (Figure 2). All dogs had hypoproteinemia (ie, serum total protein concentration < 5.5 g/dL) after the second day's exercise (320 km). Mean serum albumin concentration was significantly (P = 0.003) decreased after the second, third, fourth, and fifth days of exercise (320, 480, 640, and 800 km), compared with mean concentration prior to exercise (Table 1). However, all dogs had serum albumin concentrations > 2.3 g/dL at all times.

Figure 2—
Figure 2—

Mean ± SD serum total protein concentration (reference range, 5.7 to 8.2 g/dL) in 10 Alaskan sled dogs that exercised every day for 5 consecutive days by running 160 km/d. Values obtained after 160, 320, 480, 640, and 800 km were significantly (P < 0.05) different from value obtained at time 0.

Citation: Journal of the American Veterinary Medical Association 230, 10; 10.2460/javma.230.10.1486

All dogs had hypoglobulinemia prior to exercise (Table 1), and serum globulin concentration decreased in a significant (P < 0.001) linear fashion with continued exercise. Mean serum globulin concentration was significantly decreased after the third and fifth days of exercise (480 and 800 km), compared with mean value prior to exercise. Serum globulin concentrations after the fifth day of exercise ranged from 1.0 to 2.1 mg/dL.

Serum enzyme activities and total bilirubin concentration—After each day's exercise, mean serum CK activity was significantly (P < 0.001) higher than mean activity prior to exercise, and all dogs had serum CK activities greater than the upper reference limit at these times, with values ranging from 336 to 12,420 U/L (Figure 3). Serum AST activity was within reference limits for all but 1 dog prior to exercise but was greater than the upper reference limit in all dogs after each day's exercise, with values ranging from 80 to 595 U/L. Mean serum AST activity was significantly (P = 0.019) increased after the second and third days of exercise (320 and 480 km), compared with mean value prior to exercise (Table 1).

Figure 3—
Figure 3—

Mean ± SD serum CK activity (reference range, 46 to 320 U/L) in 10 Alaskan sled dogs that exercised every day for 5 consecutive days by running 160 km/d. Values obtained after 160, 320, 480, 640, and 800 km were significantly (P < 0.05) different from value obtained at time 0; data were log transformed prior to analysis.

Citation: Journal of the American Veterinary Medical Association 230, 10; 10.2460/javma.230.10.1486

After each day's exercise, mean serum ALP activity was significantly (P < 0.001) increased, compared with mean value prior to exercise, but was within reference limits at all times (Table 1). There was a significant (P < 0.001) linear increase in serum ALT activity with exercise, and mean serum ALT activity was significantly increased after the second, third, fourth, and fifth days of exercise (320, 480, 640, and 800 km), compared with mean value prior to exercise. After each day's exercise, mean serum ALT activity was greater than the upper reference limit. No significant (P = 0.066) changes in serum total bilirubin concentration were detected during the study, and concentrations remained within reference limits in all dogs (reference range, 0.1 to 0.8 mg/dL).

SUN and creatinine concentrations—Mean serum creatinine concentration was significantly (P < 0.001) increased after the first 4 days of exercise, compared with mean concentration prior to exercise (Table 1), but none of the dogs had concentrations higher than the upper reference limit. Mean SUN concentration was within reference limits prior to exercise, but mean SUN concentration was significantly (P < 0.001) increased after each day of exercise, compared with mean value prior to exercise, and all dogs had SUN concentrations higher than the upper reference limit, except for 1 dog with SUN concentration within reference limits after the third day of exercise (480 km).

Serum cTnI concentration—For all dogs, serum cTnI concentration was less than the limit of detection of the immunoassay (0.1 ng/mL) prior to exercise. If a cTnI concentration of 0.11 ng/mL was used as the upper reference limit for healthy dogs, then exercise was considered to have resulted in high serum cTnI concentrations in all dogs, with concentrations measured after exercise ranging from 0.2 to 1.2 ng/mL. After each day of exercise, mean serum cTnI concentration was significantly (P = 0.004) increased, compared with mean concentration prior to exercise. Mean serum cTnI concentration was highest (mean ± SD, 0.56 ± 0.3 ng/mL; range, 0.3 to 1.1 ng/mL) after the first day's exercise (160 km) but was high after the second (0.29 ± 0.1 ng/mL; range, < 0.1 to 0.49 ng/mL), third (0.23 ± 0.1 ng/mL; range, < 0.1 to 0.38 ng/mL), fourth (0.34 ± 0.3 ng/mL; range, < 0.1 to 1.2 ng/mL), and fifth (0.31 ± 0.2 ng/ mL; range, < 0.1 to 0.92 ng/mL) days also. In 2 dogs, serum cTnI concentration was less than the detection limit of the assay after the second or fourth day of exercise and remained less than the detection limit for the remainder of the study. Pearson correlation analysis revealed significant positive correlations between serum cTnI concentration and the logarithm of serum CK activity (r = 0.319; P = 0.013) but not between serum cTnI concentration and serum CK activity (r = 0.170; P = 0.194).

Discussion

Results of the present study suggest that multiple successive days of endurance exercise result in significant but mild alterations in serum chemistry variables in conditioned sled dogs. The finding of significant quadratic contrasts for serum sodium, chloride, calcium, total protein, albumin, urea nitrogen, creatinine, and cTnI concentrations and for serum CK, AST, and ALP activities suggested that exercise itself, rather than exercise duration or distance traveled, had a significant affect on these variables. The finding of significant linear contrasts for serum potassium, globulin, and phosphorus concentrations and serum ALT activity suggested that exercise duration or distance had a more important effect on these variables than did exercise alone. Apart from any impact of exercise, serum CK, AST, ALT, and ALP activities and serum urea nitrogen, creatinine, and cTnI concentrations represent the balance between production and clearance. Therefore, the role of half-life and factors associated with production and clearance, such as renal blood flow, hydration, and diet, must be considered because persistent increases may not necessarily reflect continued production or leakage of the analyte.

Alterations in serum electrolyte and mineral concentrations in the present study were mostly mild and clinically unimportant. Decreases in serum calcium concentration likely were related to decreases in serum albumin concentration.1,14,17 Increases of serum phosphorus concentration possibly reflected phosphate release from working muscle or the effects of reduced renal perfusion.4,18–20

Interestingly, serum sodium concentration was unchanged or increased during exercise in the present study. Prolonged exercise has previously been associated with decreased serum sodium concentration in racing sled dogs, with these decreases having been attributed to inadequate dietary intake and renal sodium loss secondary to substantial obligatory urinary urea excretion.9,10 In contrast, hyponatremia in human athletes is attributed to extensive sodium losses in sweat and the dilutional effects associated with ingestion of sodium-poor fluids during exercise.3 Prolonged exercise increases plasma aldosterone concentration and decreases urinary sodium excretion in exercising sled dogs9,21,22; however, several factors are likely to affect serum sodium concentrations during exercise. In particular, serum sodium concentration is positively correlated with exercise intensity and is high in dogs working at high intensity.23,24 In the present study, in contrast, dogs were estimated to be working moderately hard at approximately 40% of maximal oxygen uptake and at a pace consistent with that used in previous sled dog studies. Increases in dietary protein intake reportedly increase serum sodium and urea nitrogen concentrations in sled dogs, and dietary composition (ie, sodium, protein, and water contents), which might affect sodium dynamics during exercise,25 would be expected to vary among studies. In the present study, water intake was somewhat controlled because the only water that was provided was the water mixed into the dogs’ food. However, the dogs also had frequent access to snow and ice. Because water intake was not measured, it is not possible to conclude whether water intake was adequate or inadequate, but the minimal increase in serum creatinine concentration in the dogs suggests that increases in serum sodium concentration were not attributable to dehydration.

A progressive decrease in serum potassium concentration in exercising sled dogs has been reported previously.9,10 Trained dogs have lower resting serum potassium concentrations and display less pronounced increases in serum potassium concentration during exercise.26 Low resting serum potassium concentrations may represent progressive depletion of body reservoirs or renal loss secondary to high blood aldosterone concentrations.27 However, renal potassium conservation has been demonstrated in sled dogs despite high blood aldosterone concentration, suggesting that marked renal cation conservation occurs in sled dogs during prolonged exercise.9,10 Speculatively, repeated exercise might enhance the normal hypokalemic response that can occur when moderately intense exercise ceases.28

The significant increase in serum chloride concentration observed in the present study could reflect a response to the increase in serum sodium concentration or a decrease in serum bicarbonate concentration.17 Serum bicarbonate concentration was not measured in the present study, but exercise was only of moderate intensity, and previous studies10,29 report only mild alterations in bicarbonate concentration and acid-base status in racing sled dogs. An intriguing possibility is that serum chloride concentration might have increased to compensate for the substantial decrease in plasma protein concentration because plasma protein in dogs has a considerable negative charge.30

Serum total protein concentration was substantially decreased in the present study, reflecting decreases in both albumin and globulin concentration. A decrease in total protein concentration during exercise appears to be a consistent phenomenon in racing sled dogs.1,10,11 Possible causes include increased plasma volume, exercise-associated immunosuppression, catabolism of plasma protein for energy, and protein loss via the renal and gastrointestinal tracts.

Prolonged exercise is associated with an expansion of plasma volume in human athletes and is frequently accompanied by hyponatremia and stable or increasing body weight,3,4,18 neither of which were observed in the present study. Studies10,31 suggest that sled dogs might be less prone to profound alterations in endogenous fluid dynamics during racing than human athletes. The increase in SUN concentration observed in the current study in conjunction with the minimal changes in blood creatinine concentration may be suggestive of increased protein catabolism or the greater sensitivity of SUN concentration to changes in hydration. Racing necessitates a substantial increase in caloric intake in sled dogs, and a substantial amount of the increased calories come from protein.2,16 Additionally, racing sled dogs may undergo attenuated muscle glycogenolysis during repeated exercise, increasing reliance on other substrates.16 It is possible that endogenous protein sources such as plasma proteins might be catabolized to meet this demand.

Calculated serum globulin concentrations in the present study were less than those previously reported for dogs with protein-losing enteropathy.32 Pronounced decreases in serum globulin concentration in response to training and racing have been documented previously in sled dogs and human ultramarathon runners.4,10,13 Endurance exercise is associated with immunosuppressive effects in highly trained athletes, and hypoglobulinemia in resting dogs may partly reflect decreased production secondary to exercise-induced immune dysfunction. For example, repeated heavy exercise has a cumulative negative effect on salivary IgA concentration in human athletes.33,34

Prolonged exercise can induce proteinuria in dogs and people.35,36 However, microalbuminuria has been documented in Beagles during swimming but not running, and microalbuminuria was not documented in sled dogs from this kennel that completed a similar 560-km simulated race, suggesting that renal protein loss is minimal in running sled dogs.35,37 On the other hand, both simulated and competitive sled dog racing have been associated with development of gastrointestinal tract lesions, including bloody diarrhea, gastric erosions, and increased intestinal permeability.37,38 Given that all dogs in the present study became similarly hypoproteinemic and that albumin did not appear to be selectively lost, it is likely that a combination of factors are involved in the decrease in total protein concentration associated with repeated exercise in racing sled dogs.

Increases in serum CK and AST activities are consistent with musculoskeletal damage associated with training and racing. An increase in serum CK activity during the early stages of prolonged exercise followed by a gradual decrease despite continued exercise is a recognized phenomenon in dogs and humans participating in long-distance events.4,11 Although the reasons for this pattern are unknown, they might include adaptation of the muscle tissue to the imposed demands, changes in gait, decreased production of CK, or increased clearance of CK.4 The positive correlation between the logarithm of serum CK activity and serum cTnI concentration likely reflects the similar effect of exercise on both variables. Although specific isoenzymes of CK were not measured in the present study, it is possible that an increase in the cardiac CK fraction contributed to the observed increase in serum CK activity. However, the relatively modest increase in cTnI concentration and the lack of clinical evidence of cardiac disease suggest that release from skeletal muscle was the major contributing source.

Serum ALT activity can reflect hepatic and muscular damage in dogs.17 Given the substantially longer half-life of ALT, compared with CK and AST, it is possible that the persistent increase in serum ALT activity in the present study represents musculoskeletal injury. However, ultramarathon runners have been reported to have increases in serum ALT activity that correlate poorly with AST and CK activity, as well as pronounced increases in glutamate dehydrogenase, G-glutamyltransferase, and ALP activities.4,39 Increased dietary protein content has been associated with higher ALT activity in sled dogs, and serum G-glutamyltransferase activity was higher in dogs finishing a long-distance race than in dogs that were retired from the race, suggesting that prolonged exercise and dietary protein intake may influence hepatic enzyme activities in exercising sled dogs.12,40 Other tests of hepatic function (eg, serum bile acids or blood ammonia concentration) would be required to determine the functional impact of repetitive prolonged exercise on the liver.

Transient increases in concentrations of the various cardiac troponin fractions have been documented in highly trained human endurance athletes in response to strenuous competition and are often accompanied by corresponding ECG abnormalities.6,41 Highly trained sled dogs have echocardiographic and ECG findings that are substantially different from findings for healthy nontrained dogs, indicating cardiac adaptation to heavy training loads.42,43 However, in the present study, increases in cTnI concentration associated with exercise were documented in all dogs, and values were comparable to values reported for dogs with cardiomyopathy or degenerative mitral valve disease.15 Fluctuations in serum cTnI concentration were observed in 7 of the 10 dogs, suggesting repeated release of cTnI, rather than delayed clearance of troponin released during the initial bouts of exercise. It is likely, however, that in vigorously exercising healthy sled dogs, troponin release is a normal process not necessarily reflective of clinically important or irreversible cardiac injury.44

In conclusion, our results suggest that in response to prolonged endurance exercise, well-conditioned Alaskan sled dogs will develop multiple mild aberrations in serum chemistry values, including changes consistent with mild skeletal and cardiac muscle damage. The limited severity of these aberrations likely reflects successful adaptation to repetitive prolonged endurance exercise and appropriate dietary management. Hypoproteinemia, particularly hypoglobulinemia, is a prominent finding in repeatedly exercised sled dogs and possibly reflects a combination of immunosuppression, increased protein catabolism, and protein loss. The observed aberrations in serum protein concentration warrant further investigation regarding etiology and potential impact, including susceptibility to disease and vaccine response in heavily exercising sled dogs.

ABBREVIATIONS

cTnI

Cardiac troponin-I

CK

Creatine kinase

AST

Aspartate aminotransferase

ALP

Alkaline phosphatase

ALT

Alanine aminotransferase

a.

Eukanuba adult large breed premium performance, the Iams Co, Dayton, Ohio.

b.

IAMS original formula small biscuits, the Iams Co, Dayton, Ohio.

c.

Eukanuba maximum calorie/canine dry formula, the Iams Co, Dayton, Ohio.

d.

Full spectrum fat, the Iams Co, Dayton, Ohio.

e.

Ektachem 750 XRC analyzer, Eastman Kodak, Rochester, NY.

f.

Providence Alaska Medical Center, Anchorage, Alaska.

g.

Immulite tropinin I, Diagnostic Products, Los Angeles, Calif.

h.

SAS, version 9, SAS Institute Inc, Cary, NC.

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